Double organ-blowing bellows use two feeder chambers below a common reservoir. As one feeder fills from the room, the other pushes air into the reservoir. The upper chamber stores air between delivery peaks and supplies the organ through its wind trunk. Hiscox illustrates this arrangement as mechanism 623.
Double Organ-blowing Bellows Interactive Calculator
Explore two alternating feeder bellows below a common weighted reservoir. Enter the displacement of each feeder and complete rocker cycles per minute to estimate the pair’s average delivery.
Equation Used
- Equal swept volume and volumetric efficiency for both feeders.
- Each feeder delivers once per complete rocker cycle.
- Ideal one-way switching of inlet and delivery flaps.
- Estimated delivered volume is proportional to chamber compression displacement.
- The reservoir demonstration assumes sufficient stored air and constant mean demand.
- Zero cycle rate stops automatic movement without disabling position inspection.
Both feeders deliver once per complete rocker cycle. The calculated difference from ideal delivery is not a separately measured leakage rate.
Follow the two feeders
The external rocker drives two connecting rods. Each rod moves a feeder’s bottom board. When that board descends, its chamber expands and the inlet flap in the board opens upward. The delivery flap into the reservoir stays shut.
When the board rises, the chamber compresses. Its inlet flap seats and the upper flap opens into the common reservoir. The other feeder is then taking in its next charge. At a motion reversal, instantaneous displacement flow falls to zero.
The reservoir’s weighted lid can descend while the organ draws air and rise as the feeders replenish it. The displayed lid movement follows a volume-balance demonstration with constant outlet demand equal to average feeder delivery. Its travel scale is illustrative; it is not a prediction of wind pressure.
Compare displacement and pumping cadence
Enter the swept volume of one feeder, not the combined volume of both. Enter complete rocker cycles per minute: each complete back-and-forth cycle gives one delivery stroke from each feeder.
The volumetric efficiency is an assumption supplied by the user. It reduces the ideal geometric displacement to an estimated delivered volume. The difference can include leakage and incomplete filling; the calculator does not measure an actual leak.
Average delivery from the pair
With each feeder sweeping Vs litres and completing N compression strokes per minute, the pair’s ideal delivery is 2VsN L/min. Applying an assumed volumetric efficiency ηv gives Q=2VsNηv, with ηv expressed as a fraction.
Divide L/min by 28.316846592 for cubic feet per minute, or by 60,000 for m³/s. The difference between ideal and estimated delivery is 2VsN(1−ηv).
The animation’s instantaneous feeder delivery follows the compression portions of the actual board-position curves. The buffer-volume change is cumulative estimated feeder delivery minus constant outlet demand accumulated over the same part of the cycle. Over a complete cycle that change returns to zero. This is a displacement-balance model without a compressible-flow or pressure solution.
Two 12-litre feeders at 40 cycles per minute
Ideal delivery is 2×12×40=960 L/min. At an assumed 85% volumetric efficiency, estimated delivery is 816 L/min, approximately 28.8 CFM or 0.0136 m³/s.
The difference from ideal displacement is 144 L/min. Doubling feeder volume or cycle rate doubles these flow results. Setting cycle rate to zero gives zero flow and stops automatic movement; the position slider still lets you inspect the mechanism.
Average flow is not pressure stability
The pair feeds the reservoir on alternate half-cycles, but delivery is still pulsating. The moving boards slow to zero speed at reversals. The graph shows those delivery dips alongside the assumed mean outlet demand.
Reservoir capacity, lid loading, fold geometry and organ demand determine how a real installation responds. The illustration includes weights to identify the pressure-loading principle, but it does not size them or guarantee a particular pressure tolerance.
The rods and rocker retain their geometric lengths. Bellows outlines, fold details and the reservoir height scale are schematic. The specified feeder volume calibrates displacement; it does not supply manufacturing dimensions.
Double-bellows questions
What counts as one cycle?
One complete back-and-forth motion of the rocker. Each feeder delivers once during it.
Why does each feeder need two valves?
One admits room air during expansion; the other delivers to the reservoir during compression.
Why is there a reservoir?
It can supply the outlet while feeder delivery is below demand and accept surplus delivery at other points.
Does the model calculate organ wind pressure?
No. It calculates displacement flow and an illustrative volume balance.
Is the efficiency a measured property?
No. It is an entered assumption, and the resulting average flow is an estimate.
References
Hiscox, Mechanical Movements, Powers, Devices and Appliances, mechanism 623, printed page 168 (PDF page 178), alternating feeders below an equalizing section.
Organ Building, printed page 105, feeder and reservoir flap-valve operation and weighted reservoir board.
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